The present invention relates generally to a collet chuck assembly for holding a workpiece and more particularly to a fixed length collet chuck assembly in which the collet only translates radially and not axially.
Chucks are utilized in machining operations wherein a workpiece must be securely gripped and rotated at high speed. Examples of such chucks are disclosed in U.S. Pat. Nos. 4,349,207; 4,676,516; 4,999,002 and 5,344,166, all issued to Anton Fink. In such machining operations, chucks are provided which are adapted to be rotated by a drive means such as a lathe, and include radially movable jaws or collets for gripping and releasing a workpiece. The chuck bodies are generally cylindrical in configuration with the master jaws or collet being disposed at one end thereof and being capable of movement between a radially inward closed position and a radially outward open position. In use, a workpiece is placed parallel to the longitudinal, central axis of the chuck body and the jaws or collet are closed radially inwardly about the workpiece. The rotation of the chuck body and the workpiece enables the latter to be machined. When the machining operation is completed, the jaws or collet are urged into a radially outward open position releasing the workpiece.
Collet chucks are typically more accurate and have greater gripping characteristics than a typical jaw chuck. One such collet is disclosed in U.S. Pat. No. 4,214,766 to Rall et al. Collets are generally sleeves or collars used for clamping or gripping workpieces or tools. Collets have conical surfaces or cams, which, when the collet is pulled back, interact with corresponding, opposing surfaces or cams on a mounting fixture or on the spindle. This interaction causes the collet to contract or expand to grasp or release a workpiece or tool, depending on the direction of movement. An advantage of collets is that they continue to grasp the workpiece or tool even at high rotational speeds when jaw chucks would have a tendency to loosen their grip due to centrifugal force.
While collets have been noted for holding parts concentric, they are not very accurate for holding part lengths. Slight variations in the diameter of the workpiece or stock could cause the collet to position the workpiece (differently. When and where a collet will grasp a workpiece depends on the difference in diameter between the open collet and the diameter of the workpiece. When a chucking diameter is smaller than a setup piece, the finished part will be longer than the setup piece. The reverse is also true; if the chucking diameter is larger, the finished part will be shorter than the setup piece. Precise workpiece diameter is therefore required if the workpiece is be positioned precisely and consistently in machining operations such as facing, side finishing, or cutting to precise lengths. In the alternative, a compensation system can be added to the machine but are costly and add complexity to an already complex mechanism.
To eliminate the need for compensation systems and for precise diameter workpieces, a fixed length collet chuck was designed as manufactured by MicroCentric Corporation of Plainview, N.Y. and disclosed in MicroCentric's product catalog, page 12. In this design, a pushing force is applied to close the jaws of the collet instead of the drawing action of a conventional collet chuck. A standard collet chuck body is assembled with a fitted taper seat. As the draw tube pushes forward, the taper seat moves forward clamping the collet on the workpiece in a radial direction with no axial movement. Other examples of collet chucks which control part lengths are disclosed in U.S. Pat. No. 5,330,224 to Terwilliger et al. and U.S. Pat. No. 5,480,164 and 5,855,377 to Murphy.
While eliminating the control length problem, this fixed length collet chuck does not maximize the efficiency of the chucking system since it operates in a direction opposite of the machine spindle. Also, the prior art fixed length collet chuck is not compatible with the new servo stop type bar feeders as manufactured by SMW Systems of Santa Fe Springs, Calif. These new bar feeders have a servo mechanism which electronically centers the feed position and feeds the bar out to a pre-set position. The pushing actuation of the prior art fixed length collet chuck tends to move the bar away from the servo stop that has been pre-set resulting in varying lengths of the finished product.
It is an object of the subject invention to provide a fixed length collet chuck in which the collet translates radially and not axially upon a workpiece.
It is another object of the subject invention lo provide a fixed length collect chuck which is actuated by a pulling motion in the direction toward the machine spindle.
A further object of the subject invention is to provide a fixed length collet chuck which exerts a greater clamping force.
It is a further object of the subject invention to provide a fixed length collet chuck compatible with a servo stop type bar feeder.